These two share a number of things in common:
#1: Both are six episode (25-29 minutes per episode) OVAs.
#2: Both are based on two different 15 volume manga series created by Katsura.
#3: Both are romantic comedies, and both share many of the same character types. (FYI, a typically pathetic/weak-willed rom-com lead, a tomboy and a reserved 'dream girl'.)
#4: Both are high quality productions; the artwork remaining faithful to Katusra's wonderful, ecchi style.
#5: Both end each episode with extras, though the I''s Pure extras are far better than Ai's--the Pure extras focusing on its lead's perverted fantasies from the manga.
#6: Both are incredibly under-rated, by MAL's shitty standards. If Giant Robo is worthy of over 8/10, these two are most certainly not worthy of close to 7/10 averages.
#7: Both have been released on DVD in the US: Ai was released years ago by Geneon, and I''s + Pure got released by Viz last year.
Also, I can't comment on how good of an adaptation Ai's OVA is, but the I''s Pure OVA is as good as you can expect a six episode adaptation of a 143 chapter manga to be. The execution (starting in the present, two years in, and then going back to certain points in the I''s story) was perfect and mostly only meaningless content got cut.